With developments of integrated circuit (IC) fabrication technology, critical dimensions (CD) of an IC device have decreased continuously. As semiconductor devices become more tightly arranged, requirements for electrical isolation between active regions and devices have correspondingly become more stringent.
In existing electrical isolation technology, shallow trench isolation (STI) structures have advantages including small isolation area, super latch-up protecting ability, almost-zero field corrosion, very small active region tilt angle, beneficial for one-step process, etc. Thus, as microelectronic processes enter deep-sub-micron stage, the STI structures are the most commonly-used electrical isolation structures.
An existing process for forming STI structures includes the following steps. First, a photo mask is formed on a semiconductor substrate. Next, using the photo mask as an etch mask, the semiconductor substrate is etched to form a trench. Finally, the trench is filled with an insulative material to form the STI structure.
In the STI forming process, a dry etching process is the most commonly-used process. The dry etching process can result in an anisotropic etch profile, and may have desired sidewall profile control and CD control. A trench formed by the dry etching process can have a desired trench structure. An etching effect of the dry etching process is achieved by chemical actions, physical actions or a combination of both.
The dry etching process usually includes the following steps. First, an etching gas is excited into a plasma by a high-frequency electric field. Next, for chemical actions, the plasma generates reactive elements including free radicals and reactive atoms. For physical actions, the plasma generates energetic ions including positive ions. The reactive elements can react with the semiconductor substrate, and etch the semiconductor substrate. The energetic ions can be accelerated by an electric field vertical to a surface of the semiconductor substrate to bombard the semiconductor substrate, and remove a surface material of the semiconductor substrate using sputtering etch.
In the dry etching process, reactive elements or energetic ions generated by the plasma can react with the semiconductor substrate to form the trench. At the same time, a large amount of by-products are produced. These by-products adsorb a large amount of charged particles, and attach to inner walls of the trench, which can affect insulation properties of the subsequent-formed shallow trench. Therefore, during the dry etching process, inert gases are supplied and circulated in order to remove the by-products gathered on the trench surface and on a bottom surface of the semiconductor substrate.
With developments of semiconductor technology, an aspect ratio of the shallow trench formed in the semiconductor substrate has gradually increased. Thus, it has become increasingly difficult to clean up the by-products on the inner walls of the shallow trench. Existing cleaning process usually cannot effectively clean up the by-products inside the shallow trench, which can lead to defective electrical isolation of the subsequently-formed STI structure.